EUV light source

ABSTRACT

An EUV light source apparatus and method for producing EUV light, which includes a plasma generation chamber for generating EUV plasma; an EUV light collector having a reflective portion irradiated by EUV light produced in the EUV plasma; a target sample having reflective portion comprised of the same materials as the EUV light collector reflective portion; a first EUV detector for detecting EUV light produced in the EUV plasma; and a second EUV detector for detecting EUV light reflected from the target sample.

RELATED APPLICATIONS

The present application is a divisional of U.S. application Ser. No.10/900,839, filed Jul. 27, 2004, which is a continuation-in-part toco-pending application Ser. No. 10/803,526, entitled A HIGH REPETITIONRATE LASER PRODUCED PLASMA EUV LIGHT SOURCE, filed on Mar. 17, 2004, andSer. No. 10/798,740, entitled COLLECTOR FOR EUV LIGHT SOURCE, filed onMar. 10, 2004, and is related to Ser. No. 10/742,233, entitled DISCHARGEPRODUCED PLASMA EUV LIGHT SOURCE, filed on Dec. 18, 2003, and Ser. No.10/409,254, entitled EXTREME ULTRAVIOLET LIGHT SOURCE, filed on Apr. 8,2003, and Ser. No. 10/189,824, entitled PLASMA FOCUS LIGHT SOURCE WITHIMPROVED PULSE POWER SYSTEM, filed on Jul. 3, 2002, all of which areassigned to the common assignee of the present application, thedisclosures of each of which are hereby incorporated by reference.

FIELD OF THE INVENTION

The present invention relates to the generation of EUV light, e.g., atthe power levels required for EUV integrated circuit lithography withthe required dose stability and other parameters that will be necessaryfor such uses.

BACKGROUND OF THE INVENTION

As discussed in the above referenced co-pending applications one aspectof an EUV light source operating at ten to twenty thousand pulses of EUVlight per second, or even higher, and using, e.g., a moving target,e.g., a mass limited droplet, is the ability to track the position andtiming of the targets and their respective arrival at a desired plasmainitiation site. This involves, e.g., determining a spot in 3D spacewhich is imaged by an EUV light collector to an intermediate focus (IF),e.g., at an exit point for the EUV light from an EUV light generationchamber containing the collector and the desired initiation site. Alsoas discussed in the above referenced co-pending applications a dropletdelivery system, including, e.g., a droplet generator and aiming system,needs to be aligned so that the droplets are projected through or fallthrough (in the case of a gravity feed) the spot that constitutes thedesired plasma initiation site; corresponding to the focus of thecollector and a small area around this focus, e.g., ±10 μm in which EUVgenerated from a laser produced plasma will still be adequately focusedto the intermediate focus of the system, a so-called desired plasmainitiation region around the desired plasma initiation site. Alsorequired is to be able to fire the laser to have the drive laser beamintersect the target droplet at the desired plasma initiation site,i.e., when a droplet arrives exactly at the desired initiation site. Itwill be understood, as noted, that the desired initiation site may varyslightly from the precise focus of the collector, e.g., at a first focusof an elliptical collector mirror having a second focus comprising theintermediate focus of the light source system, e.g., by about 10 μm for,e.g., droplets of about 10 μm-40 μm in diameter and still be in focusenough for adequate collection. Therefore, a function of the trackingsub-system is not only to determine when to fire the laser(s) but atwhat selected plasma initiation site if not the desired plasmainitiation site at the true focus, and the corrections necessary for thedelivery system to, in the meantime, bring the target delivery to thedesired plasma initiation site. The system may also determine that thetarget droplet is not on a track to arrive at the desired plasmainitiation site and that, therefore there will not be any effectivegeneration of EUV light that will arrive at the intermediate focus, andthat therefore the laser should not be fired while the target dropletsare returned to a proper target track to intersect the desired EUVplasma initiation site. Alternatively, laser firing could be allowed tocontinue even though adequate EUV light is not being collected while thetarget positioning is ongoing.

It will be understood that, while “desired plasma initiation site” asused herein is the focus of the collector, some area around the focus ofthe collector in which aiming the drive laser beam at a so-calledselected plasma initiation site that is slightly off of the collectorfocus, can still be effective for generating an effective amount of EUVlight at the intermediate focus (“the desired plasma initiationregion”). “Selected plasma initiation sites” that are not on thecollector focus, but within the desired plasma initiation region, havingan acceptable distance error in both the x and y planes, as definedbelow, may occur. In the event that the laser will continue to be firedeven if the selected plasma initiation site is outside the desiredplasma initiation region, then selected plasma initiation sites mayoccur outside the desired plasma initiation region also.

Aspects of performing these functions have been discussed in the abovereferenced co-pending applications. Applicants herein propose certainimproved apparatus and methods for accomplishing these functions.

Applicants have developed unique approaches to place the targets, e.g.,individual Li droplet targets in the right position in 3D space, aiminga laser beam at a droplet position and firing the laser at the rightmoment in order to better enable operation of EUV LPP source, accordingto aspects of embodiments of the present invention. The irradiation ofthe target, e.g., a target droplet, heats the droplet sufficiently tocause the formation of a plasma through, e.g., evaporation/ablation andphotons in the laser beam strip off electrons forming ions of evaporatedtarget metal atoms in the plasma, and in this sense the target isignited at a plasma initiation site, using the meaning of ignite orignition to mean the subjecting of the target to intense heat and/or toheat up or excite, and generally meaning the formation of the plasmafrom the irradiated target due to the impartation of the heat (energy)from a drive laser beam intersecting the target and igniting the targetto form a resultant plasma, that in turn produces EUV radiation. The useof the term ignition in the above referenced applications will beunderstood to have this meaning. Another meaning for ignition is theheating of a plasma to a temperature high enough to sustain nuclearfusion. While likely that such a temperature is attained in the plasmaformation according to aspects of the present invention, which, however,involves none of the attempts to confine the plasma so formed accordingto aspects of the present invention sufficient to induce and/or sustainfusion, the conception of an ignition of a plasma according to aspectsof an embodiment of the present invention has a similar meaning as usedin the above referenced applications. In the present application thesame concept is expressed by the term “plasma initiation” and “plasmainitiation site,” meaning the irradiation of the target causes theplasma to form “plasma initiation” and this occurs or is desired tooccur at some “plasma initiation site.”

Lithium for use as a target as discussed in above referenced co-pendingapplications likely will have at least some impurities in it. Evenlevels of impurities in the parts per million range, over time, cancause unwanted and damaging depositions within an LPP EUV chamber, e.g.,on the collector optics and/or various chamber windows. Theseimpurities, contained in an LPP target droplet of liquid lithium, afterplasma initiation will be deposited, e.g., on the collector mirror.Since many of these impurities have much higher boiling temperaturesthan the, e.g., 400-500° C. proposed collector temperature, e.g., toevaporate the deposition of lithium itself, it is more difficult toremove these impurities from the collector using the previouslysuggested evaporation techniques. Applicants in the present applicationsuggest a way of dealing with this problem in previously proposed LPPand/or DPP EUV chamber components, e.g., the optical components.

As discussed in prior co-pending applications referenced above, thecollector needs to operate at an elevated temperature (e.g., at least atabout a range of 400-500° C.), e.g., in order to evaporate Li from itsreflective surface and maintain its reflectivity. Applicants propose inthe present application apparatus and methods to maintain a stable anduniform temperature range across the optics of the collector over whichits performance is able to meet required specifications, e.g., theavoidance of collector distortion due to maintenance of the elevatedtemperature.

Utilization of a solid state laser, e.g., a Nd:YAG laser to drive a LPPEUV source, with 1064 nm laser light is often doubled, has been known toemploy doubled, tripled, etc. frequencies, e.g., to possibly achievehigher conversion efficiency at smaller wavelengths produced at thefirst harmonic generation (“FHG”) and second harmonic generation(“SHG”). This has been based on accessing a higher density plasma layerwith the shorter wavelength higher harmonics, such that more sourceatoms are available for excitation and subsequent emission. Ingenerating the higher laser harmonics, however, a large fraction(perhaps 30-50% for SHG and 80% for FHG to 266 nm) is lost because it isnot converted in the nonlinear crystals.

Applicants have also developed, according to aspects of embodiments ofthe present invention ways to achieve higher conversion efficiency fromlaser energy converted to EUV radiation, and which allows extremelyprecise control of the initial density scale length, which will allowprecision optimization of the laser deposition of energy into a target,e.g., a droplet, for improved conversion energy output ratios.

One of the problems in focusing optics for EUV LPP sources with Li orsimilar elements is a contamination and degradation of the optics due tocontamination from Li or other elements. Applicants have developedaccording to aspects of embodiments of the present inventionutilizations of grazing incidence optics or other EUV radiationcollection optics for the improvement of conversion efficiency.

Also an issue in systems of the type of aspects of an embodiment of thepresent invention relates to the need for protecting optics other thanthe collector, e.g., windows and focusing optics, which may be combined,e.g., in introducing the drive laser beam into the EUV light sourceproduction chamber, which are addressed in the present application.

SUMMARY OF THE INVENTION

An apparatus and method for EUV light production is disclosed which maycomprise a laser produced plasma (“LPP”) extreme ultraviolet (“EUV”)light source control system comprising a target delivery system adaptedto deliver moving plasma formation targets and an EUV light collectionoptic having a focus defining a desired plasma initiation site,comprising: a target tracking and feedback system comprising: at leastone imaging device, e.g., a digital video or motion picture cameraproviding as an output an image of a target stream track, a stream trackerror detector detecting an error in the position of the target streamtrack in at least one axis generally perpendicular to the target streamtrack from a desired plasma initiation site, which may include the focusof the collector and an area around that focus within which plasmainitiation may still produce acceptable level of EUV light at an EUVlight output, e.g., the intermediate focus in an elliptical collectormirror system. At least one target crossing detector may be aimed at thetarget track and detect the passage of a target through a selected pointin the target track. A drive laser triggering mechanism utilizing anoutput of the target crossing detector to determine the timing of adrive laser trigger in order for a drive laser output pulse to intersectthe target at a selected plasma initiation site along the target trackwithin the desired plasma initiation region. A plasma formation detectormay be aimed at the target track and detecting the location of theactual plasma initiation site for a respective target and, e.g.,position vis-a-vis the focus of the drive laser beam. An intermediatefocus illuminator may illuminate an aperture formed at the intermediatefocus to image the aperture in the at least one imaging device. The atleast one imaging device may be at least two imaging devices eachproviding an error signal related to the separation of the target trackfrom the desired plasma initiation site at the collector focus basedupon an analysis of the image in the respective one of the at least twoimaging devices. A target delivery feedback and control system maycomprise a target delivery unit; a target delivery displacement controlmechanism displacing the target delivery mechanism at least in an axiscorresponding to a first displacement error signal derived from theanalysis of the image in the first imaging device and at least in anaxis corresponding to a second displacement error signal derived fromthe analysis of the image in the second imaging device. An EUV outputlight energy detection mechanism may comprise a plurality of EUV lightenergy detectors disposed to measure EUV light energy originating fromthe plasma initiation site, each providing an output signalrepresentative of an amount of EUV light energy detected by therespective EUV light energy detector; an EUV light energy error signalgenerator receiving the output of each EUV light energy detector anddetermining an EUV light energy error signal based upon a comparison ofthe respective values of the output signals of the respective EUV lightenergy detectors. A laser irradiation timing error detection mechanismmay comprise use of the EUV light energy error signal to determining atleast a timing factor of a positioning error of the laser beam vis-a-visthe target droplet position at the time of plasma initiation. A plasmaproduced extreme ultraviolet (“EUV”) light source collector may comprisea plasma initiation chamber; a shell within the plasma initiationchamber in the form of a collector shape having a collector focus; theshell having a sufficient size and thermal mass to carry operating heataway from the multilayer reflector and to radiate the heat from thesurface of the shell on a side of the shell opposite from the collectorfocus. The material of the shell may be selected from a groupcomprising: silicon carbide, silicon, Zerodur or ULE glass, aluminum,beryllium, molybdenum, copper and nickel. A heat sink adjacent the shellon the side of the shell opposite from the focus absorbing heat radiatedfrom the adjacent surface of the shell may be provided. A laser producedplasma (“LPP”) extreme ultraviolet (“EUV”) light source may have a drivelaser producing a drive laser output pulse beam and a drive laser outputpulse beam directing system and an EUV light collector having a focus,and may comprise a beam focusing system intermediate the beam directingsystem and the collector focus, operative to focus the output laserpulse beam to a selected position in the vicinity of the collectorfocus. The beam focusing system may comprise a focusing lens and areflective focusing element intermediate the focusing lens and thecollector focus and having a focusing lens focal point intermediate thefocusing lens and the reflective focusing element; and the reflectivefocusing element focusing the beam at the selected position. Opticelement debris mitigation in such systems may be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows aspects of an embodiment of the present inventionillustrated schematically;

FIG. 2 shows a side view of aspects of the present invention illustratedschematically as in FIG. 1

FIG. 3 shows further details of the schematic illustrations of aspectsof an embodiment of the present invention illustrated in FIGS. 1 and 2;

FIG. 3A shows a block diagram of an EUV metrology system according toaspects of an embodiment of the present invention;

FIG. 4 shows schematically an in situ lithium purification systemaccording to aspects of an embodiment of the present invention;

FIG. 5 shows aspect of an embodiment of a collector according to anembodiment of the present invention:

FIGS. 5A-5C show an alternative to FIG. 5 according to aspects of anembodiment of the present invention;

FIGS. 6A-6D show further aspects of an embodiment of an LPP EUV systemaccording to aspects of an embodiment of the present invention;

FIG. 7 is an illustration of the relationship between the distancebetween the laser produced plasma and the intermediate focus accordingto aspects of the present invention as a function of illuminator inputsolid angle, for collector diameters of 450 mm (5π sr collection angle)and 250 mm (2π sr collection angle);

FIG. 8 is an illustration of the relationship between the heat load inW/cm² and the collector mirror diameter for a 5π sr collector and acomparison to the approximate heat load from solar radiation incident onthe earth, i.e., ≈14 W/cm²;

FIG. 9 is an illustration of emissivity as a function of mirror diameterat 400° C. and 500° C.;

FIGS. 10A and 10B show schematically a collector with athree-dimensional physical debris shield according to aspects of anembodiment of the present invention;

FIG. 11 shows schematically and in cross section a metrology systemaccording to aspects of an embodiment of the present invention;

FIG. 12 shows schematically and in cross section an apparatus and methodfor protection of system optics from debris according to aspects of anembodiment of the present invention;

FIG. 13 shows schematically and in cross section an alternativeembodiment to that of FIG. 12 according to aspects of an embodiment ofthe present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

According to aspects of an embodiment of the present invention,Applicants propose portions of an EUV light source system 10,illustrated schematically in FIG. 1, that is capable of aspects ofactive control of the position, pointing and focusing of an EUV plasmainitiation drive laser(s) and pre-pulse laser(s) directed at a target,e.g., in a plasma formation/EUV source chamber 64 (not shown in FIG. 1),e.g., a moving droplet (20, as shown in FIG. 2) of liquid metal, e.g.,lithium, which may be mass limited.

Some general requirements according to aspects of an embodiment of thepresent invention include the need to collect as much EUV light from theLPP as possible according to which applicants presently contemplate aneed for about a 5 sr solid angle for collection of the plasma producedEUV light. In addition, contemplated is to provide a matching angle tothe illuminator with a need for the correct solid angle for acceptanceat the intermediate focus 42 (“IF”), e.g., about 0.038 sr. Presentlycontemplated is the use of an ellipsoid as the geometrical shape for thecollector 40, which can be related to a working distance w from a plasmaformation point, e.g., at a desired plasma initiation site, e.g., at thefocus 32 of the collector 40, a collector 40 diameter D, or a selecteddistance between the LPP desired plasma initiation site and the IF, orsome combination thereof. With, e.g., such an ellipsoidal design for aprimary collector 40, a working distance may be, e.g., 200 mm, an outerdiameter may be, e.g., driven by collection and acceptance angles and,for 5 sr collection angle/200 mm working distance this may imply theneed for, e.g., a substrate OD of, e.g., 622 mm for such an exemplary0.038 sr acceptance angle. Also currently contemplated is a single shelldesign, with a monolithic substrate making thermal and opto-mechanicaldesign considerations easier, although for other than concept proof,more complex compositions and geometries may be required.

Turning now to FIGS. 1 and 2 there are shown aspects of an embodiment ofthe present invention. The applicants propose a system 10 in which laserproduced plasma (“LPP”) extreme ultraviolet (“EUV”) light sourcetargets, e.g., droplets 20 of liquid lithium, at the rate of 10-20thousand per second, or even higher rates, and traveling at velocitiesof about 20 m/sec and of a size of about 10 μm in diameter, or possiblyhigher, e.g., up to around 40 μm, can be tracked to within accuracies ofless than the diameter of the target 20 and calculated to be located ata selected initiation site 30 (e.g., as close on the track of the targetdroplet 20 to the actual desired initiation site, e.g., at the focus 32of a collector mirror 40 at a specific time, in order to fire a drivelaser 50 (not shown) to produce a drive laser beam 52 to intersect thetarget droplet 20 at the selected initiation site with about a 50-100 nstiming tolerance. It will be understood that the desired plasmainitiation site corresponds to the focus 32 of the collector, but thatsome error, e.g., within a sphere of about a 10 μm radius about thefocus of the collector (“the desired plasma initiation region”) maystill produce effective amounts of EUV light at the intermediate focus42. As such the system 10 may be configured to aim the laser beam at aselected plasma initiation site which is not the desired plasmainitiation site but within this acceptable positioning error region, thedesired plasma initiation region, while the system is concurrentlycorrecting the track of the targets and the positioning of the drivelaser beam at selected plasma initiation sites to eventually be at thedesired plasma initiation site 32, i.e., at the focus of the collector40. It will be understood that if the target track is sufficiently inerror in any axis to not allow for plasma initiation within the desiredplasma initiation region, the system 10 may block the triggering of thelaser until this condition is corrected by the system 10, oralternatively allow continued firing of the laser at selected plasmainitiation sites outside the desired target initiation region while thetarget tracks and laser pointing are moved to place the selected plasmainitiation site within the desired plasma initiation region andeventually at the desired plasma initiation site.

To accomplish these requirements applicants propose to provide imagingequipment, e.g., digital video or motion picture imaging equipment,e.g., two imaging devices 60, 62, which may be, e.g., digital camerashaving a digital output representative of the video intensity of eachpixel in the camera's field of view, e.g., between 0 and 256 shades ofgray. The cameras 60, 62 may be CCD cameras. The cameras 60, 62 may havea frame rate as is typical for current video cameras, e.g., 30 framesper second, though other frame rates may be utilized as well accordingto aspects of embodiments of the present invention. The cameras 60, 62may be focused, e.g., using a cylindrical lens (not shown) in two planesintersecting the target droplet 20 line of flight (e.g., target deliverystream 92 shown in FIG. 3) from a release point, e.g., at a targetdelivery system 80 outlet 82, as shown in FIG. 2, or the camera may beotherwise lensed. Each of the two cameras 60, 62 may be placed, e.g., toview along the track of the droplet 20 from the delivery system 80 tothe selected initiation site 30 and including in the view the desiredplasma initiation site 32 as well, and oriented, e.g., 90° from eachother to detect the position of the respective target track 92 in,respectively, an x axis and a y axis, each generally orthogonal to therespective target track 92 from the target delivery system 80 to thedesired plasma initiation region around the desired plasma initiationsite 32.

This is illustrated in more detail in FIG. 3 where the field of view ofthe respective camera, e.g., camera 60, includes an image of the targetstream 92 (above the desired plasma initiation site 32) and 92′ belowthe desired plasma initiation site 32, each of which may be slightlydifferent in intensity (e.g., shade of gray) due to the fact that thestream 92 has more droplets 20 in it before plasma formation by some ofthe droplets 20 than the stream 92′ after plasma formation by some ofthe droplets 20.

It will be understood that the terms above and below and horizontal andvertical are used throughout this application only illustratively andcoincide with those directions as illustrated in the drawings forillustrative purposes only. The directions and orientations may bedifferent in actual operation, e.g., the droplets may be directed to thedesired plasma initiation site 32 by imparting a velocity to the droplet20 at the target supply 82 and shooting the targets 20 at the desiredplasma initiation site 32 as opposed to utilizing a purely gravity feed,in which event the stream need not be “vertical.” Also shown in theschematic illustration of FIG. 3 is the image 90 in the camera 60 fieldof view of the intermediate focus, illustrated as a circle or oval 90.It can be seen, as illustrated in FIG. 3 that the stream 92, 92′ may beslightly off in the respective horizontal axis from passing through thedesired plasma initiation site 32, the detection and quantification ofwhich from the image data of the respective camera 60, 62 may be used todirect the target delivery system to redirect the target delivery stream92 and may also be used to direct a drive laser to a selected plasmainitiation site for a next to be irradiated droplet 20, according to theactual track of the flight of the droplets 20 and whether or not, e.g.,that track passes through a desired plasma initiation region around thedesired plasma initiation site 32 at the focus of the collector 40. Itwill be understood that the image in the second camera 62 may besimilarly used for control of the stream track 92 in a second axis,e.g., the x axis, so that the selected initiation site 30 can be movedto the desired initiation site 32, which coincides with, e.g., a focusof an elliptical collector mirror 40, so as to thereby focus thereflections from the collector mirror 40 of the EUV produced by targetirradiation at a selected plasma initiation site 30 to the IF at asecond focus of the elliptical collector 40 mirror system, and move theselected plasma initiation site 30 to the desired plasma initiation site32.

Also illustrated in FIG. 3 schematically is the utilization of a pair ofcontinuous wave, e.g., HeNe lasers to illuminate droplets 20 as theypass by selected positions in the target track 92 intermediate thetarget delivery system 80 and the desired plasma initiation site 32. Thelaser beams 108 from the HeNe lasers may be tightly focused into aselected plane, e.g., by cylindrical lenses 104, 106 and theintersection of a droplet 20 with first the beam from laser 100 and thenthe beam from laser 102 may be detected by, e.g., photo-detectors 120,122, respectively, through, e.g., focusing lenses 110, 112. In this way,the successive flashes indicative of the respective droplet intersectingpoint 94 and then point 94′ may be detected as flashes of light by thedetectors 120 and 122 respectively. It will be understood that lasers100 and 102 may be, e.g., focused at a separation distance, e.g., equalto the droplet 20 separation or the detection circuitry suitably timedto discriminate between droplet crossings of different droplets 20.Also, it will be understood that, assuming a known or empiricallydetermined droplet speed, only one laser, e.g., laser 100 and onedetector 120 may be needed to determine the arrival timing of arespective droplet 20 at the selected plasma initiation site forpurposes of timing the firing of the respective drive laser.

The other of the cameras 62 may have a field of view oriented in a planeperpendicular to the first camera 60. The two cameras 60, 62 can thus beused to triangulate in three dimensions the positions of moving targetdroplets 20. Each of the two cameras 60, 62 may have in its field ofview an image 90 of the intermediate focus created by the collectormirror 40 represented by the oval 90 on CCD camera image. The center ofthis oval 90 can be considered to coincide with the desired plasmainitiation site 32, i.e., where ideally plasma initiation should occur,i.e., at the focus 32 of the collector mirror 40. With the cameras 60,62 working at, e.g., 30 frames per second, a relatively slow speedcompared to the droplet velocity and the droplet repetition rate, e.g.,10,000-100,000 per second, the cameras 60, 62 will see those droplets 20as a continuous stream 92 as illustrated in FIG. 3.

Alignment of these lasers 100, 102 on the stream 92 of droplets may bedone using the two CCD cameras 60, 62, e.g., when the He—Ne laser lightis focused right on the stream 92, the cameras 60, 62 will see a brightspot on the stream 92, which is centered on the stream 92. If the laserbeam from the respective laser 100, 102 is slightly off, the bright spot94, 94′ will be off as well.

Two photo-detectors 120, 122 may each be used to look at light fromrespective laser 100, 102 reflected from the droplets 20 as they passthrough the, e.g., tightly focused beam of these two He—Ne lasers 100,102. Each of these detectors 120, 122 can then generates a pulse everytime a droplet 20 passes through the timing spots 94, 94′, respectively,of the corresponding He—Ne laser 100, 102. The detectors may also havefilters and the lasers 100, 102 operated at different wavelengths, tofacilitate the discrimination in the flashes from spots 94 and 94′respectively. The pulses generated by these photo-detectors 120, 122 maythen be used, e.g., to calculate the speed and expected timing of thedroplet 20 arrival at a selected plasma initiation site 30, which liesalong the stream 92. A drive laser 50 can then be aimed at the selectedplasma initiation site 30, e.g., along the stream 92, e.g., which isstill within the desired plasma initiation region, and fired tointersect the droplet 20 at the selected plasma initiation site 30 at acalculated time of arrival of the droplet at the selected plasmainitiation site 30.

A plasma created by the irradiation of the droplet target 20 at theselected initiation site 30 due to the absorption by the droplet 20 ofthe energy in the laser beam pulse 52 from the drive laser 50 will thenalso be visible as a bright plasma initiation image spot 96 on the CCDimages of cameras 60, 62. Proper filters can be used to adjust theintensity of this plasma initiation image spot 96 on the CCD cameras 60,62. This plasma initiation image spot 94 can be an indication of wherethe drive beam laser beam 52 was focused on the target droplet 20.

Horizontal alignment of the drive laser beam 52 may be done, e.g., byaiming the drive laser beam 52 plasma formation plane (which may beselected to be before the focus plane or after the focus plane of thelaser beam 52 according to operating parameter requirements influencedby the selection of the plasma formation plane of the laser beam 52)and, e.g., in the horizontal plane of the selected plasma initiationsite 30 in the desired plasma initiation region. The bright spot at theselected initiation site 30 may be centered in the middle of the stream92 of droplets 20. For illustrative purposes, again recognizing thathorizontal and vertical are for illustration only and coincide with theviews of the drawings, the horizontal plane may be considered to containthe x and y axes of a coordinate system, generally orthogonal to adirection of target movement from the target droplet 20 supply towardthe desired plasma initiation site 30, while the z axis is generallyaligned with this direction of travel, and is in any event orthogonal tothe horizontal plane. Because, according to aspects of an embodiment ofthe present invention, the drive laser(s) 52 may not be aimed at everydroplet 20, there can also be a stream 92′ of droplets 20 downstream ofthe selected target spot 30 as well. With this vertical alignment of thelaser beam 52 done, e.g., by placing the bright spot 94 in the middle ofthe IF image oval/circle 90, i.e., at the horizontal axis, for any givenx-y axes error(s) in the position of the selected initiation site 30relative to the desired initiation site 32 at the focus of the collectormirror 40, will be the closest point that the droplet 20 will approachthe desired initiation 32 site along path of the stream 92, and withinthe desired plasma initiation region around the desired plasmainitiation site 32.

A feedback control loop may be used to walk the, e.g., two streams 92seen respectively by the two cameras 60, 62 to center each at animaginary centerline axis of the intermediate focus aperture, e.g., byhorizontal translation of the target delivery system 80 in thehorizontal plane x and y axes, to move the selected initiation site 30to the desired initiation site 30. That is, to move the imaged streams92 in the x-y plane to each intersect the desired plasma initiation site32, and at the same time the system may be moving the laser aiming pointto continue to intersect, e.g., the stream at a selected plasmainitiation site along the image of the stream 92, to intersect a dropletat the selected plasma initiation site.

More precise timing adjustments of the drive laser 50 firing time may bedone, e.g., to place the arrival of the pulse in the laser beam 52 fromthe drive laser 50 at the selected (and eventually desired) initiationsite 32, 30 with the target droplet 20 fully within the beam formaximized irradiation of the target droplet 20, e.g., by a feedback loopthat optimizes the EUV power produced in each plasma formation. Adithering technique can be used, e.g., dithering the trigger signaltiming, to, e.g., converge on a maximum EUV plasma output. The EUVdetectors measuring the balance of the EUV radiation at pointssurrounding the desired plasma initiation site 32, e.g., in one planearound the desired plasma initiation site 32, can also be used for inputin adjusting the laser firing timing (trigger signal timing) to, e.g.,the travel of a target droplet along the droplet stream path 92 towardthe plasma initiation site 30, 32.

An alternative or supplemental technique for precise laser timingadjustment and horizontal alignment may use, e.g., four EUV detectors154 placed outside of the collector 40. According to aspects of thisembodiment of the present invention, e.g., when the laser beam isaligned horizontally and vertically to the selected/desired plasmainitiation site, and properly timed, the signals from all four of thedetectors 154 will be the same, indicating that the intersection of theentire target droplet with the laser beam 52 is occurring, and thusproperly timed. The output of these detectors may be utilized asexplained in more detail in regard to FIG. 3A in determine when totrigger the drive laser 50 or a respective one of the plurality of drivelasers 50 to optimize the irradiation of a target droplet 20.

Turning now to FIG. 3A there is illustrated in block diagram schematicform an exemplary metrology system 150 according to aspects of anembodiment of the present invention. The system 150 may comprise, e.g.,the camera 60, 62 providing inputs in digital form to an imageprocessing module 152, which may process the images from camera 60independently from those of other camera, e.g., camera 62.

The image processing module 152 may provide an output indicative of HeNeflash intensities to a HeNe pointing control module 170, which may,e.g., translate horizontally the respective HeNe laser 102, 104 tomaximize the intensity of the flashes detected in the camera image infeedback control, e.g., to properly point the HeNe lasers at the stream92.

The image processing module 152 may also provide a position error signalfor each camera image, e.g., between the actual plasma initiation site32 and the center of the IF aperture image, e.g., in imaginaryhorizontal and/or vertical planes, to a source laser control module 172.The source laser control module may utilize this information fromrespectively camera 60 and camera 62 for source laser aiming control in,e.g., the y and z axes.

The image processing module 152 may also provide, e.g., a droplet streamerror signal indicative of the error between the stream path 92 for arespective camera 60, 62, with respect to an imaginary verticalcenterline axis of the IF aperture image in each of the x and y axes asindicated respectively from the image in camera 60 on the one hand andcamera 62 on the other, i.e., through the desired plasma initiation site32. This can then be used by the droplet control module 174 to generatex, y axis feedback control signals to the droplet delivery system 80.

The outputs of the photo-detectors 120, 122 may be utilized to provideinput signals to a laser trigger control module 154. The laser triggercontrol module 154 may use the timing between the detected flashes fromdetector 120 and 122 respectively as an indication of the droplet 20speed along the path 92 toward a selected initiation site 30, also alongthat path 92, and, as noted above within the desired plasma initiationregion, so as to be, e.g., as close to the desired initiation site 32 aspossible. This may be utilized then to produce a trigger signal to asource drive laser control module 180 to fire the source laser 50 totime the arrival of the source laser pulse 52 at the selected initiationsite 30 along the path 92 at the same time the target droplet 20 arrivesat the selected plasma initiation site 30 within the desired plasmainitiation region around the desired plasma initiation site 32.

Aspects of EUV light source apparatus and methods include, among otherthings, optical design, opto-mechanical design, thermal engineering,substrate selection and fabrication, multi-layer reflective coatingdevelopment and fabrication. As discussed in co-pending applicationsmentioned above, it is currently contemplated to operate a collectormirror inside of an EUV generation chamber 64 at a temperature ofbetween about 400-500° C., with the objective of, e.g., evaporatinglithium in a liquid-lithium target system to, e.g., preserve thereflectivity characteristics of the mirror multi-layer reflectivitystack forming near normal angle of incidence EUV reflective mirrors ofthe collector 40 for collection of the generated EUV radiation, e.g.,from a plasma generated using a drive laser(s). It will be understoodthat near normal angle of incidence includes angles of from 0° and about45°, between the incident EUV radiation and the normal to the mirrorsurface, for which, e.g., multilayer reflective coatings formed bystacks of many tens of layers to one hundred or so layers allow forreflection of light at EUV wavelengths, e.g., at 13.5 nm, whilereflecting surfaces of uncoated material or having a reflective coatingof only, e.g., two layers, may be used to reflect light at EUVwavelengths, but only at so-called grazing angles of incidence, whichthose skilled in the art will understand to be up to about 70°-90° fromthe normal, depending on wavelength, reflective material, coatings andthe like.

A number of contemplated designs exist for a collector mirror 40, e.g.,including a simple ellipsoidal mirror having, e.g., a focus desiredplasma initiation site 32 at one focal point of the ellipse and theintermediate focus 42 of the collector mirror 40 at the other focus ofthe ellipse, comprising an entry point from the EUV light generator 10into a utilization tool, e.g., an EUV integrated circuit lithographytool. This design may result in the inability to collect a substantialfraction of the photons emitted from the plasma, depending upon a numberof factors, including the direction that the plasma actually emitsphotons, given the drive laser irradiation geometries and techniquesactually employed, among other factors. Another possibility is aspherical primary collector mirror or an ellipsoidal primary collectionmirror with grazing incidence secondary focusing mirrors.

Regardless of the selection for the collector optics, some provisionwill need to be made for protecting the optics from debris generatedwithin the EUV plasma formation vessel/chamber 64 as has also beendiscussed in co-pending applications referenced above.

Applicants address in the present application aspects of an embodimentof the present invention, which for convenience of illustration will beexplained in reference to an illustrative assumed collector geometry,i.e., that of the spherical primary collector mirror 40, e.g., with asolid angle to the IF of between 0.03-0.20 sr, a maximum collectormirror 40 outer diameter (“OD”) of 45 cm, a minimum collector 40 OD of25 cm, and a collection angle of from 5π-2π sr. Regardless of collector40 geometry ultimately selected, the collector optics (mirror 40 and theother possible elements) must maintain shape at operating temperature.Also for purposes of illustration this is selected to be in a range of400-500° C., or even higher, e.g., up to about 700° C. The selection ofa spherical geometry simplifies somewhat the illustrative calculationsreferred to in the present application, but the principles enunciated inthis application are applicable as well to ellipsoidal or other, e.g.,hyperbolic and other conic geometries. Table 1 includes otherillustrative factors and assumptions relating to designing, e.g., anillustrative thermal management system for an EUV light source chambercollector 40.

TABLE 1 Calculate Heat Load on Ø45 cm Mirror: In Band 13.5 nm EUV Powerat IF (W) 115.00 Transmission of Buffer Gas 0.90 In Band 13.5 nm EUVPower from Mirror (W) 127.78 Mean Reflectivity of Mirror 0.50 EUV PowerOnto Mirror (W) 255.56 Mirror Collection Angle (sr) 5.00 EUV Power into2π (W) 321.14 Conversion Efficiency (into 2π, in band) 0.03 Plasma Powerinto 4π (W) 10,704.69 Plasma Power onto 5 sr Mirror (W) 4,259.26Spherical Radius of 5 sr Mirror with OD = 45 cm (cm) 22.99 Surface Areaof 5 sr Mirror with OD = 45 cm (cm²) 2,642.70 Heat Load on 5 sr Mirror(W/cm²) 1.61 Mirror shell has two surfaces 0.81 Stefan-Boltzmann: IdealBlack Body Radiation at 400° C. (W/cm2) 1.16 Ideal Black Body Radiationat 500° C. (W/cm²) 2.02 Mean Emissivity Required @ 400° C. 0.69 MeanEmissivity Required @ 500° C. 0.40

TABLE 2 Material Reaction Sintered Bonded Single Crystal Silicon SiliconCVD Silicon Zerodur ULE Aluminum Property Beryllium Silicon CarbideCarbide Carbide Molybdenum (Schott) (Corning) 6061-T6 Nickel Thermal210.0 124.0 110.0 150.0 250.0 138.0 1.5 1.3 180.0 82.9 Conductivity, k(W/m · K) Thermal 11.6 2.5 4.4 1.9 3.5 5.1 1.0E−07 1.0E−05 23.6 13.3Expansion Coefficient, α (ppm/K) @ 20° C. Specific Heat @ 1886 702 — —640 276 821 776 896 471 20° C. (J/kg · K) Yield Stress (MPa) 380 120 552307 450 550 49.8 276 59 Elastic Modulus 303 112.4 400 393 466 320 80.367.6 68.9 207 (GPa) Max Service — — 1650 1400 >1400 — <500 <500 — —Temperature (° C.) Melting Point (° C.) 1283 1412 2610 N/A 652 1453Density (g/cm³) 1.848 2.329 3.1 3.1 3.2 10 2.53 2.21 2.7 8.9 LiCompatibility Risk Poor Risk Risk Risk Good Poor Poor Poor Risk

Selection of materials for the collector 40 body 220 involvesconsideration of ultra high vacuum (“UHV”) compatibility, serviceabilityat a temperature >500° C., good thermal stability, high thermalconductivity, low coefficient of thermal expansion, high strength, gooddimensional stability, particularly at elevated operating temperature,and for, e.g., normal or grazing incidence angle reflection thecapability of being polished to an extremely high quality figure andfinish. Such may include obtaining high quality figures and finish,e.g., approximately 4 Å, e.g., using high spatial frequency rougheners(“HSFR”). Also important can be the ability to bond to other materials.Table 2 refers to some materials and illustrative properties.

With regard to multi-layer reflective coating stacks for normal angle ofincidence reflection the following considerations need to be taken intoaccount, i.e., high temperature stability needed for LPP with Li,interlayer diffusion of multi-layers, reflectivity degradation atelevated temperature, impact of exposure to Li and Li compounds,sputtering from high energy Li particle impact or other materials e.g.,high energy ions and other debris, implanting and diffusion of Li intomulti-layers, selection of capping layer as a barrier, othercontaminants on the mirror surface and sputtering of other materialsfrom other components inside chamber.

It will be understood by those skilled in the art that according toaspects of an embodiment of the present invention the present inventionincludes, e.g., an optical design may comprise an ellipsoidal primarycollector 40, which may have, e.g., a working distance of 200 mm from anEUV plasma initiation site 32 to the collector 40 reflective surfacecomprised of a reflective multi-layer reflective coating. The size ofthe outer diameter of the collector 40 may be driven, e.g., bycollection and acceptance angles, e.g., for a 5 sr collection angle/200mm working distance, a substrate outer diameter of about 622 mm could beneeded for, e.g., a 0.038 sr acceptance angle.

Considering a single shell design, without the inclusion of efficiencyimprovements and tradeoffs which could become more complex, and designs,including, e.g., auxiliary collectors could also add to the complexity,according to aspects of an embodiment of the present invention thepresent invention may be considered to utilize, e.g., a monolithicsubstrate.

This can enable, for evaluation purposes, easier thermal andopto-mechanical designs, e.g., utilizing the above referenced materialsin Table 2, e.g., with SiC or Ni or other materials, e.g., Mo, Be andSi.

According to aspects of embodiments of the present invention an opticalshell 220 capable of collecting and redirecting the EUV light to asecond, or “intermediate” focus 42 is provided, as shown in FIG. 5. Theshell 220 may be made, e.g., of a suitable material, which may beconstructed to have a significant thermal mass. Thermal mass is afunction of both heat capacity and thermal conductivity of a structureof a given material. Heat capacity is a measure of how much heatdifferent types of material can hold. For a given structural element ofa given material, it is found by multiplying the density of the materialby its overall thickness, and then by its specific heat. Specific heatis the amount of heat a material can hold per unit of mass. For example,for large heavyweight materials, it can take a significant amount ofenergy to heat up their surface. This is because much of the energy isactually absorbed deeper into the material, being distributed over alarger volume. With a lot of energy incident on the surface, thisabsorption can continue until it travels through its entire width,emerging on the inside surface as an increase in temperature. Thisconduction process can take a significant amount of time. If the energyincident on the surface of one side fluctuates, this can set up “waves”of temperature flowing through the material.

Suitable materials may include but are not limited to silicon carbide,silicon, Zerodur or ULE glass, aluminum, beryllium, molybdenum, copperand nickel.

The collector shell 220 may be exposed to heat flux from the laserproduced plasma, which results in radiative heat loads such as thosereferenced above in Table 1. Such thermal loads, combined with thethermal mass of the shell 220, then result in a steady state temperatureof the shell 220, which may, e.g., be less than the operatingtemperature required for evaporating the source element (Li) debris fromthe reflective surfaces of the collector 40. This is so since theemissivity of the radiating surfaces of the optic 40 combined with itstemperature can result in the ability to radiate more thermal energythan is deposited by the laser produced plasma.

Additional heat flux to maintain the required operating temperature ofthe optic 40 may then have to be provided, e.g., by auxiliary radiantheaters 230. These radiant heaters 230 may be placed in suitablelocations in front of or even behind the body 220 of the collector 40and may need to have a directional capability, e.g., using a directionalcontrol mechanism (not shown) to accommodate the need for directing moreor less radiation on the collector 40 or on certain areas of thecollector 40 surface. Applicants now believe that approximately 5 kW ofpower or less may be needed to maintain a steady state temperature ofabout 500° C. on the collector body 220. Also presently contemplated isthe ability to maintain temperature uniformity within 2° C. or better.

To maintain control of the desired operating temperature of thecollector 40 a cooling system 250, which may include a cooling device232, may also be needed. This cooling device 232 may, e.g., be utilizedto absorb radiated heat from the rear side 234 of the collector body 220of the collector 40. It is possible that this cooling device 232 can bea simple open loop cold surface behind the rear surface 234 of thecollector body 220 of the collector 40 and may be gas or liquid cooled,e.g., with helium or water and maintained at some nominal temperature.If the power levels are low enough it is contemplated presently that itwill be possible that the cooling may be entirely radiative without theneed for, e.g., liquid or gaseous coolant, e.g. from a heat exchanger(not shown) for the cooling device 232. The emissivity of the rearsurface 234 of the collector body 220 of the collector 40 may, e.g.,then be arranged by design to be as high (close to unity) or as low asrequired. The design choices made for collector 40 geometry can, e.g.,have a significant impact on the details of this cooling system 250. Thebasic concept, however, can accommodate many variations of designassuming the body 220 of the collector 40 is large enough to reradiateincident heat loads with a practically achievable value of emissivity onkey surfaces.

It is also understood by applicants that the plasma emission at and fromthe LPP plasma formation is non-uniform. Applicants currently believethat the emission is in the form of a cosine or like distribution ratherthan an isotropic distribution. Applicants also currently understandthat the thermal radiative energy distribution is also non-isotropic andprobably also of the shape of the light emission, e.g., cosine as justnoted. Such a non-isotropic distribution can, e.g., differentially heatthe collector 40 and, e.g., more specifically the collector body 220 andcollector 40 reflective optical surfaces, which can result intemperature related deformations or figure errors on, e.g., thecollector 40 optics. Significant variations in temperature across thereflecting optics surfaces can lead to focus errors in the projection ofthe collector 40 optics, and/or, e.g., in the light-utilizing tool.Applicants propose to resolve this problem by correcting and managingvariations in temperature across the reflector optics by differentiallyheating the collector 40, e.g., with zoned heaters. Such heaters may,e.g., be powered and controlled differentially to radiate differingamounts of heat to discrete areas of the collector reflective optic.Thus, if thermally induced deformations produce differentiallydistributed figure errors on the collector 40 optic surface these can becorrected by differentially heating the collector 40 with a suitableheater array.

An exemplary heater array and collector cooling system 250 is shownillustratively and schematically according to aspects of an embodimentof the present invention in FIG. 5A. The embodiment according to FIG. 5Ashows heater elements 240 illustrated schematically to be, e.g., crosssections of heater element wires positioned, e.g., in some geometricallydefined surface 242 intermediate the collector body 220 and the coolingelement 232, serving to heat the collector body 220 as needed and alsodifferentially, e.g., zonally as explained in more detail below withrespect to FIGS. 5B and 5C.

Turning to FIGS. 5B and 5C there is shown, respectively, a perspectiveview and a perspective cross-sectional view, of a coolingelement/internal heat exchanger 232 that may be constructed, e.g., froma copper substrate formed of, e.g., C110 copper, and formed, e.g., froma first thin plate 252 and a second thicker plate 254 which may bebrazed together at a joint (not shown) with the second thicker plate 254having machined into it radial cooling galleries 256, which may beinterconnected for fluid passage between the respective galleries, butneed not be. The brazed joint (not shown) can be stronger than thecopper itself. Also possible are other forms of joinder, e.g., diffusionbonding. The combination plate assembly 232 may then be shaped, e.g., bypunching or dye pressing to form the appropriate shape to conform to thegeneral shape of the back side of the collector body 220.

C110 is almost pure copper with a thermal conductivity of 388 W/mK and a10 ksi yield stress making it relatively ductile and relatively easilyformed into the shape to match the rear side of the collector body 220.In the assembly, e.g., all vacuum wetted joints may be, e.g., highintegrity brazed joints. As shown in FIG. 5 B, the so formed coolingelement 232 may form a structure for mounting heater elements 240, 240′on radially extending ceramic spacers 244.

Referring now to FIG. 5C there is shown in cross-sectional perspectiveisotropic view a collector and temperature control assembly 222according to aspects of an embodiment of the present invention. Thetemperature control assembly 222 may include, e.g., the collector body220 and cooling element 232 along with the heater assembly 238. Theassembly 222 may include a sealing bracket 262, which, as illustrated,may be circular and may be attached to the side walls of the LPP chamber64 by a side wall mounting ring (not shown). The sealing bracket 262 mayhave formed in it a circular sealing slot 265 to receive a sealingflange 264 on a sealing ring 260 which may serve together to form aseal, e.g., a labyrinth seal to prevent, e.g., lithium or other plasmasource medium material, or other debris from reaching the copper of thecooling element 232.

The sealing ring 260 may be attached to a mounting ring 270 byattachment nuts 272. The attachment ring 270 may be attached to amounting flange (not shown) e.g., formed at the terminus of at least onedrive laser beam inlet passage 282 (not shown in FIG. 5C) by mountingring nuts 274. A sealing ring (not shown) adjacent the mounting flange(not shown) at the terminus of the drive laser beam inlet passage 282(not shown in FIG. 5C) may serve to hold an annular ring 236 on thecooling element 232 in place between an annular shelf 276 on themounting ring 270 and the mounting flange (not shown).

The heater assembly may be formed of heater elements 240 forming aspaced apart quadrant grouping and/or heater elements 240′ formingcircular elements. Each of the elements 240, 240′ may be separatelyactivated with current through wiring (not shown) and controlled by acontroller (not shown) to effect the desired differential heating, e.g.,zonal heating. The ceramic spacers 244 may keep the heater elements 240,240′ in the space between the collector body 220 and the cooling element232 and out of contact with either.

The collector body 220 may be formed to have an annular outer holdingring 288 held in place by a plurality of holding assemblies spacedaround the circumference of the collector body 220 and formed of a pairof compression balls 266, one of which, e.g., in the mounting ring 270,may be spring biased with a spring 268 to firmly hold the collector body220 in place, e.g., between the sealing ring 260 and the mounting ring270, but not overly stressing the relatively brittle, e.g., glass,material of the collector body 220. It will be understood that thecollector 40 optic, e.g., a multilayer stack forming a reflecting opticmay be formed on or mounted on the collector body 220 spaced internallyof the annular holding ring 288.

Some of the advantages of the design shown illustratively in FIGS. 5Band 5C are that heating the high emissivity back surface of thecollector 40 body 220 may be more efficient than the heatingcontemplated in the design illustrated in FIG. 5. In addition, thereflectivity of the collector 40 optics, if any, to infrared radiation,is not an issue with the assembly as proposed in FIGS. 5A-5C. Further,the sealing of the cooling element 232 from the LPP plasma formationchamber 64 removes the issue of protecting the heater elements 240, 240′and/or the cooling element 232 from, e.g., the plasma source mediummetal, e.g., lithium. The differentially controllable heater elementsand their positioning concentrically and/or radially and/or inquadrature arrangements and selectively energizing the elements, mayserve to better control the uniformity of the heating of the collectorbody 220 as required, as well as differentially compensating, ifnecessary, for differentially heating of the collector body 220 from theplasma production side of the collector 40. Furthermore, the heaterelements 240, 240′ do not compete for space in the EUV collection pathnor interfere with placement of, e.g., target droplet distributionand/or tracking, or plasma formation tracking or like equipment andsubsystems. The entire assembly 222 can also serve to shieldopto-mechanical components from high temperature in the plasma formationchamber.

Applicants currently contemplate that some form of SiC as shown in Table2 may have the best mix of properties for the collector body 220substrate since there would exist, e.g., a low risk for coefficient ofthermal expansion (“CTE”) mismatch with reflective multilayer coatings.SiC is, however, a hard material and, therefore, time consuming topolish, though it may be Si clad and diamond turned and then polished.Due, e.g., to construction considerations, however, other materials suchas Ni, e.g., Ni plating on other lighter materials, Mo., Be or plain Simay be useful. CTE issues may, however, be dominant with some or all ofsuch other materials.

FIGS. 7-9 show graphs relating to the design of a heat management systemfor the LPP EUV light source collector 40 according to aspects of anembodiment of the present invention.

According to aspects of an embodiment of the present invention asillustrated in FIG. 6A an LPP drive laser beam 52 used as a radiationsource for the target illumination for an LPP EUV source could be usedin a vacuum chamber 64. This chamber 64 may, e.g., have a window 284 forthe beam 52 and this window 284 may be, e.g., placed far from the lasertarget interaction zone including a desired initiation site 32. Focusingoptics 300, e.g., a grazing incidence optic 302, made for example ofrefractive metal like Ru or others, may serve to focus the beam 52 onthe interaction zone desired plasma initiation region around the desiredplasma initiation site 32 (i.e., including selected plasma initiationsites 30 [not shown in FIG. 6A] if different from the desired site 32,but within the desired plasma initiation region, as discussed above).The focusing optic 300 may focus the beam 52, after the beam 52 has,e.g., been passed through a focusing lens 54 with a focal point betweenthe lens 54 and the focusing optic 302, such that the rays in the beam52 are incident upon points along the surface, e.g., an ellipticalsurface 302, of the focusing optic 300, including, e.g., nested suchsurfaces as shown in the art. The light reaching the grazing optic 300surfaces 302 may thus appear to have come from a source at the focus ofthe focusing lens 54 intermediate the focusing lens 54 and the grazingincidence optic 300. The grazing incidence optic 300 may also bereplaced by normal incidence reflecting optics as well. The opticalelement 302 may be elliptical, hyperboloid, ovate, parabolic, sphericalor the like or combinations thereof, and functions to better focus thebeam 52 at the plasma initiation site 32, 30 in the interaction zoneinside the chamber 64. A small aperture, e.g., in a separation wall 304,which can be about 1 mm in diameter, may be used, e.g., to block targetdebris material and other debris material from reaching the focusinglens 54.

According to aspects of an embodiment of the present invention,therefore, aspects of EUV collection optics have been adapted forfocusing laser radiation on the selected plasma initiation site 30,e.g., within the desired plasma initiation region around the desiredplasma initiation site. All the protection schemes like heating andevaporation, etc., and others including those discussed below, then canbe used for this optic 300 also.

According to aspects of an embodiment of the present invention,illustrated in FIG. 6B a parabolic reflector surface 310 forming areflector 300′ shell may be used to focus the beam 52. As illustrated inFIG. 6C a combination of surfaces, e.g., ellipses in series or a Wolterreflector comprising a combination of a paraboloid 320, followed by aconfocal and coaxial paraboloid 322 can be used to focus the beam 52 inthe vicinity of the focus of the collector 40 (not shown in FIGS. 6A-D),i.e., at a selected plasma initiation site within the desired plasmainitiation region. In FIG. 6D the beam 52 is passed through a flat orcurved optic 330 and onto a flat or curved reflecting surface 332 tofocus at the collector focus 32. For example for a curved optic 330,the, e.g., flat optic reflecting surface 332 is between the focal pointof the optic 330 and the optic 330 itself and the flat optic 332 focuseson the collector focus 32. The flat optic 332 may be, e.g., a part of aconical laser beam input passage 570 (shown in FIG. 12).

Turning now to FIGS. 10A and 10B there is shown, respectively,schematically a side cross-sectional view along cross sectional lines10A in FIG. 10B, and a front view, of a collector 40 with a debrisshield of the collector 40 and foil separators 500, 502 in the debrisshield. By way of example the collector 40 may form an ellipticalreflecting surface, symmetric about an axis of rotation with the foilseparators 500, 502 of the debris shield intermediate a focus 32 of theelliptical reflecting surface. The foil separators 500, 502 may, e.g.,comprise alternating long foil separators 500 that may extendessentially from the drive laser beam 52 opening in the collector 40 anddebris shield to a radial extent of the debris shield, and short debrisshield foil separators 502 that extend, e.g., from the radial extent toa position intermediate the radial extent and the drive laser beam 52opening. The thin foil separators 500 may comprise foil sheets that areas thin as structurally possible and, together with the also thin aspossible short foil sheets 502, may form light passages that are alignedwith EUV ray paths from the focus 32 to the collector 40 reflectingsurface and back to the intermediate focus 42. The intermixture of longfoil sheets 500 and short foil sheets 502 may serve, e.g., to increasethe light passage openings without significantly detracting from eitherstructural integrity or the debris removal function of the foil sheets500, 502. It is also contemplated that the foil sheets can be ofmultiple lengths, i.e., the short foil sheets may themselves be ofdifferent lengths and distributed in some pattern about the axis ofrevolution along with intervening long foils sheet separators oralternatively randomly so distributed.

It will be understood that the foil sheets 500, 502 may serve to plateout lithium or other target metal and/or compounds thereof, includingcompounds of impurities, e.g., introduced in the lithium targetmaterial, whether compounds with the target material itself orotherwise, that may otherwise reach the collector 40. The foil sheetseparators 500, 502 may also have other debris mitigation impacts.Further incoming material, e.g., lithium and lithium ions may sputtermaterial from the foil separator sheets 500, 502. The collector debrisshield separator foil sheets 500, 502 may be heated to evaporate some ofthe materials plating out on the separator foil sheets 500, 502 afterplating on the separator foils sheets 500, 502.

RF coils 510 may form a plasma barrier intermediate the debris shieldand the plasma initiation at the focus 32 to, e.g., slow down andscatter, e.g., fast moving ions and/or debris of other forms, so thatsuch ions or debris end up deposited on the foil sheets 500, 502.Magnetic fields creates by, e.g., steering magnets 512, 514, which maybe permanent magnets or electo-magnets, may serve to steer, e.g., ions,induced by the LPP plasma or in the RF induced plasma in front of and/orin the area of the debris shield, to be turned away from the collector40 and its sensitive multi-layer reflecting surfaces.

Turning now to FIG. 11 there is shown schematically and in cross-sectionan exemplary EUV energy detection system and collector efficiencymetrology system that may comprise, e.g., an EUV power detector (powermeter) 162′ in an EUV reference measurement arm 518 which may, e.g., beintermittently exposed to EUV light, e.g., originating at the plasmainitiation site in the desired plasma initiation region around thedesired plasma initiation site 32, by the operation of a shutter 520.Also part of the EUV collector efficiency metrology system may be, e.g.,a collector sample 530 disposed in the chamber 64 at a location thatwill not significantly block light passage from the collector 40 (notshown in FIG. 11) but positioned to receive plasma debris relatively inthe same amounts as, e.g., the actual multi-layer reflecting surfaces onthe collector 40, and, e.g., made of the same materials. The collectorsample 530 may also be protected from debris in ways to simulate thesame protections, if any, implemented to protect the actual collector 40reflecting surfaces and/or calibrated in some fashion to account for thedifferences in debris exposure over that of the actual collector 40reflecting surfaces.

It will be understood that the collector efficiency metrology system mayoperate by exposing the EUV power meter 162′ in the EUV referencemeasurement arm 518 to the EUV emanating from the plasma at the plasmainitiation site 32, e.g., by opening the shutter 520 and at the sametime reading the EUV power meter 162″ at the end of the EUV measurementarm 522. This can, e.g., give the difference between the EUV generatedat the plasma initiation site 32 and the EUV reflected from thecollector sample 530 to the detector 162″. This may, from the outset bedifferent values due to, e.g., the reflectivity losses in themulti-reflective coatings on the collector sample 530, the same way thatthe collector itself reflects less than all of the light emitted fromthe plasma initiation site that reaches the collector 40 reflectivesurfaces. Over time, however, the change in this difference may reflectadverse impacts of the operation of, e.g., debris formation on, thecollector 40. This may be utilized to calibrate other aspects ofmetrology measurements for the overall system, e.g., dependent uponcollector 40 performance, e.g., EUV received at the IF or in thelithography tool, e.g., from the change(s) in time of the differencebetween the EUV light sensed at the EUV detector 162′ and 162″. It willbe understood that the detector 162′ may also serve as part of thearrays of detectors 162 shown in FIG. 1 as explained above to detect,e.g., the geometric balance of the EUV energy generated at the plasmainitiation site 32 for purposes of detecting, e.g., drive lasermis-timing in irradiating a respective target, e.g., a respective target20.

Turning now to FIG. 12 there is shown schematically and in cross sectiona form of debris management system to protect, e.g., the drive laserfocusing optic, which may, e.g., form a drive laser input window 54 in adrive laser beam input passage 282. The drive beam input passage 282 mayhave surrounding it at one end toward the EUV plasma initiation site 32a plasma formation mechanism, e.g., a plasma formation RF coil 540 whichin operation may serve, e.g., to ionize or further ionize debris, e.g.,plasma source medium, e.g., target metal, atoms and/or ions, e.g.,lithium and lithium compound and lithium impurity compound, atoms and/orions, that enter the laser beam input passage 282, e.g., through thedrive laser beam opening in the collector. The ions, formed at the EUVplasma initiation site and/or formed or further energized in the plasmainitiation RF field formed in the laser beam passage 282 by the RF coils540, may then be steered by steering magnets 550. The RF field formed bythe coils 540 also serves to slow ions entering the passage 282 from theEUV plasma initiation region. The steering magnets 550 may form asteering magnetic field that turns the plasma in the laser beam inputpassage 282 into a debris trap 532 extending at an angle, e.g.,orthogonally to the laser beam input passage 282. The debris flow 534 sosteered, may be incident upon, e.g., a charged plate 552 which may benegatively charged to some voltage—U to plate out the debris materialcontained in the debris flow 534.

Further enhancing the protection of, e.g., the optic formed bywindow/lens 54 may be, e.g., a flow of purge gas, e.g., helium, from apurge gas inlet 560 to a purge gas outlet 562 through the drive laserinlet passage 282. Alternatively, the purge gas may be exhausted intothe chamber 64 through a conical laser beam inlet tube 570, as shownillustratively in FIG. 12, in lieu of the purge gas outlets 560 or inaddition to the purge gas outlet 562.

Turning now to FIG. 13 there is shown schematically and in cross sectionanother form of debris management system for the protection of, e.g., awindow/lens 54 in the drive laser inlet passage 282. This system of FIG.13 may comprise, e.g., a shield plate 580 having an aperture 582,through which the drive laser beam 52 may be focused on the way to,e.g., a drive laser focusing optic 300, in the drive laser input passage282, with the purge gas inlet 560 on a drive laser beam input side ofthe shield plate 580 and the purge gas outlet 562 on a collector 40 sideof the shield plate 580. The system of FIG. 13 may also have a debrisplasma formation mechanism which may comprise, e.g., RF coils 540, whichmay serve to slow down ions entering the passage from the EUV plasmainitiation region.

It will be understood that the plasma focusing optic 300 within thedrive laser input passage 282 may serve, e.g., to refocus the drivelaser beam 52 at the target plasma initiation site 32 so as to allow,e.g., a longer drive laser input passage, e.g., 1000 mm between the EUVplasma initiation site end of the drive laser inlet passage 282 and theshield plate 580, as opposed, e.g., to the 300 mm of the embodiment ofFIG. 12 between the EUV plasma initiation end of the drive laser inletpassage 282 and the optic 54. This can enable, e.g., more dissipationand collection of the debris between the EUV plasma initiation site 32end of the drive laser beam inlet passage 282 and the shield plate 580,keeping the debris at the aperture 582 at a minimum and facilitating thepurge gas flow through the aperture 582 in blocking the passage ofdebris through the aperture 582. The aperture 582 may have an opening ofabout 1 mm and be placed at the focal point of the optic 54. Laser beam56 is then focused by the optic 54 into a focal point 590 near theorifice of the aperture 582. Mirrors 302 then refocus the beam 54 intothe plasma initiation site 32. The side walls of the drive laser inletpassage 282 may be kept at a negative voltage and/or a magnetic field(s)may be used to encourage the debris to flow to and deposit on theinterior walls of the drive laser inlet passage 282 intermediate the EUVplasma initiation site end of the passage 282 and the shield plate 580.

The drive laser focusing optic 300 may also be heated electrically by anelectrical connection (not shown) in addition to any RF heating from thecoils 540, and, being reflective to, e.g., DUV of the drive laser beam52, but not EUV, even at a grazing angle of incidence, will not focusEUV or debris back to the aperture 580. The laser focusing optic 300 mayhave metal mirrors 302.

aspects of the present invention have been described

1. An apparatus comprising: an EUV light source comprising: an EUV lightsource EUV plasma generation chamber; an EUV light collector having areflective portion irradiated by EUV light produced in the EUV plasma;an EUV light collector sample having a reflective portion comprised ofthe same materials as the EUV light collector reflective portion; afirst EUV detector detecting EUV light produced in the EUV light sourceplasma; and a second EUV detector detecting EUV light reflecting fromthe EUV light collector sample.
 2. The apparatus of claim 1 furthercomprising: a comparator comparing the output of the first EUV detectorand the second EUV detector and providing a signal indicative of thedifference in the output of the first and second EUV detectors.
 3. Theapparatus of claim 2 further comprising: the EUV light collectorcomprises debris mitigation apparatus deterring debris formed in the EUVplasma generation chamber from detracting from the reflectivity of theEUV light collector; and, EUV light collector debris mitigationapparatus deterring debris formed in the EUV plasma generation chamberfrom detracting from the reflectivity of the EUV light collector samplein essentially the same manner and degree as the debris mitigationapparatus deterring debris from detracting from the reflectivity of theEUV light collector.
 4. The apparatus of claim 3 further comprising: thedifference between the output of the first EUV detector and the secondEUV detector indicating the impact of adverse effects on thereflectivity of the collector due to debris from plasma formation. 5.The apparatus of claim 2 further comprising: the second EUV detectorcomprising calibration to account for differences between the effects ofdebris mitigation on the reflectivity of the collector and the collectorsample.
 6. The apparatus of claim 5 further comprising: the differencebetween the output of the first EUV detector and the second EUV detectorindicating the impact of adverse effects on the reflectivity of thecollector due to debris from plasma formation.
 7. The apparatus of claim2 further comprising: the second EUV detector comprising: protectionfrom debris which simulate the debris mitigation protection implementedto protect the collector reflecting surfaces from plasma produceddebris.
 8. The apparatus of claim 7 further comprising: the differencebetween the output of the first EUV detector and the second EUV detectorindicating the impact of adverse effects on the reflectivity of thecollector due to debris from plasma formation.
 9. The apparatus of claim2 further comprising: the difference between the output of the first EUVdetector and the second EUV detector indicating the impact of adverseeffects on the reflectivity of the collector due to debris from plasmaformation.
 10. The apparatus of claim 2 further comprising: thedifference between an output of the first EUV detector and the secondEUV detector comprising a calibration for at least one other EUV lightdetector.
 11. The apparatus of claim 2 further comprising the first andsecond EUV detectors comprising part of an array of detectors.
 12. Theapparatus of claim 11 further comprising: an array of EUV detectors. 13.The apparatus of claim 12 further comprising: the array of EUV detectorcomprising a mechanism for measuring the geometric balance of the EUVenergy generated at the plasma initiation site.
 14. The apparatus ofclaim 13 further comprising: the measured geometric balance comprisingfeedback to control at least in part the timing and/or positioning ofthe output of a drive laser initiating plasma formation.
 15. Theapparatus of claim 1 further comprising: the EUV light collectorcomprises debris mitigation apparatus deterring debris formed in the EUVplasma generation chamber from detracting from the reflectivity of theEUV light collector; and, an EUV light collector debris mitigationapparatus deterring debris formed in the EUV plasma generation chamberfrom detracting from the reflectivity of the EUV light collector samplein essentially the same manner and degree as the debris mitigationapparatus deterring debris from detracting from the reflectivity of theEUV light collector.
 16. The apparatus of claim 15 further comprising:the debris mitigation apparatus comprising: an EUV light sourcecollector debris shield comprising: a plurality of light passagesaligned with the light emanating from a first focus of the collectormirror and reflected to a second focus of the collector mirror; thelight passages formed by adjacent thin foil separators at least some ofwhich foil separators extend radially a shorter distance than others ofthe foil separators.
 17. The apparatus of claim 1 further comprising:the second EUV detector comprising calibration to account fordifferences between the effects of debris mitigation on the reflectivityof the collector and the collector sample.
 18. The apparatus of claim 1further comprising: the second EUV detector comprising: protection fromdebris which simulate the debris mitigation protection implemented toprotect the collector reflecting surfaces from plasma produced debris.19. The apparatus of claim 1 further comprising: the difference betweenan output of the first EUV detector and the second EUV detectorcomprising a calibration for at least one other EUV light detector. 20.The apparatus of claim 1 further comprising the first and second EUVdetectors comprising part of an array of EUV light detectors.
 21. Theapparatus of claim 20 further comprising: an array of EUV detectors. 22.The apparatus of claim 21 further comprising: the array of EUV detectorcomprising a mechanism for measuring the geometric balance of the EUVenergy generated at the plasma initiation site.
 23. The apparatus ofclaim 22 further comprising: the measured geometric balance comprisingfeedback to control at least in part the timing and/or positioning ofthe output of a drive laser initiating plasma formation.
 24. A methodcomprising: operating an EUV light source comprising the steps of:providing an EUV light source comprising: an EUV light source EUV plasmageneration chamber; an EUV light collector having a reflective portionirradiated by EUV light produced in the EUV plasma; an EUV lightcollector sample having a reflective portion comprised of the samematerials as the EUV light collector reflective portion; utilizing afirst EUV detector, detecting EUV light produced in the EUV light sourceplasma; and utilizing a second EUV detector, detecting EUV lightreflecting from the EUV light collector sample.
 25. The method of claim24 further comprising: utilizing a comparator, comparing the output ofthe first EUV detector and the second EUV detector and providing asignal indicative of the difference in the output of the first andsecond EUV detectors.
 26. The apparatus of claim 25 further comprising:the EUV light collector comprises debris mitigation apparatus deterringdebris formed in the EUV plasma generation chamber from detracting fromthe reflectivity of the EUV light collector; and, utilizing an EUV lightcollector debris mitigation apparatus, deterring debris formed in theEUV plasma generation chamber from detracting from the reflectivity ofthe EUV light collector sample in essentially the same manner and degreeas the debris mitigation apparatus deterring debris from detracting fromthe reflectivity of the EUV light collector.